Field of the Invention
The invention relates to an image-sensor structure, and more particularly to an image-sensor structure with a hybrid filter.
Description of the Related Art
An image sensor is a kind of semiconductor device that transforms optical images into electrical signals. Image sensors can be generally classified into charge coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) image sensors. Among these image sensors, a CMOS image sensor comprises a photodiode for detecting incident light and transforming it into electrical signals, and logic circuits for transmitting and processing the electrical signals.
In an RGB camera module with a silicon-based image sensor such as CCD or CMOS, an IR-cut filter for cutting near infrared (IR) light is coated on a glass and assembled in a lens module. The lens module is then mounted on the image sensor. However, the IR-cut filter manufactured using multi-film interference technology causes a blue shift to occur.
In an RGB-IR camera module, the IR-cut filter is replaced with a dual band pass filter on a glass. The dual band pass filter manufactured by the multi-film interference technology presents a visible band and a narrow infrared band. For an RGB pixel, its color filters receive visible light and a little infrared light (noise). For an IR pixel, its color filters receive infrared light only.
There are two solutions currently employed to overcome the blue shift issue. One solution is the adoption of blue glass. The other solution is the adoption of a hybrid filter on bare glass. However, the minimum thickness of the blue glass is 0.3 mm. The hybrid filter on bare glass also needs the glass to be at least 0.15 mm thick. The total thickness of the camera is always a problem in smartphones. Additionally, adoption of the dual band pass filter on a glass also addresses the thickness problem.
Therefore, the development of a thin image-sensor structure that does not lead to blue shift is desirable.